Optical components, such as, for example, lasers, modulators, demodulators, etc., are often expected to function for extended periods of time in potentially damaging environments. Accordingly, such an optical component is typically enclosed in a packages that provides an acceptable operating environment internal to the package. The predictable and continued operation of component is thus ensured. However, optical signals must typically enter and exit a component via a single or multimode fiber optic cable. A hermetic seal to the fiber preserves the integrity of the inner environment of the component package. Unfortunately, known hermetic seals are not always satisfactory. Most available fiber optic hermetic seals are unduly complex, or substantially weaken the fiber such that routing the fiber inside the package is a delicate task, prone to injuring the fiber and/or rendering it more susceptible to subsequent injury in service due to vibration, temperature cycling, etc. Degradation or failure of the optical component can result.
For example, a fiber optic cable includes a core, a cladding disposed about the core, and a buffer layer disposed about the cladding layer. As is well known by those of ordinary skill in the art, the indices of refraction and the relative dimensions of the core and cladding are selected to provide the desired optical performance of the cable. The buffer layer primarily provides strength and stress relief to protect the delicate cladding and core.
One known technique for providing a hermetic seal between a package and an optical fiber is illustrated in cross section in FIG. 1. A length of the optical fiber is stripped of the buffer layer 4, leaving the cladding layer 3 and the core 2. A first hermetic sealing joint 5 seals the cladding layer 3 of the fiber to a ferrule 6. Typically the stripped length 1 of the fiber is metallized and the first hermetic sealing joint is a solder joint between the metallized cladding and the wall 8. A second solder joint 7 seals the ferrule 6 to the wall 8 of a package. An epoxy joint secures the buffer layer 4 to the ferrule 6. The length of fiber 1 available inside the package for routing to optical components includes only the cladding layer 3 and the core 2; the buffer layer is absent. Accordingly, the length 1 of fiber is prone to microcracks and other damage and routing the cable is problematic, as bends, twists, or other stress producing configurations should be avoided. Though certain components, such as laser diodes, may not require significant routing of the feedthrough fiber inside the package, other components, such as components based on lithium niobate, can necessitate such routing. Despite careful designs that avoid bends or turns, and appropriate care during assembly of the component, the fiber may fail or degrade in service. A naked fiber, i.e., one devoid of cladding, can be the weakest link in the component performance chain. Accordingly, there is a need for an improved hermetic seal for optical fibers.
It is an object of the present invention to provide an improved hermetic optical fiber feedthrough.
It is a further object of the present invention to provide a simple and economical optical fiber hermetic seal that is easily assembled.
Yet another object of the invention is to provide a optical fiber having a hermetic seal located midway between sections of fiber, where both sections include a buffer layer.
Other objects of the invention will in part be apparent and in part appear hereinafter.